can be classified in many ways. Perhaps the best classification scheme comes from Leigh and Zee (2006). In their book, The Neurology of Eye Movements, Leigh and Zee divide eye movements into the following functional classes:
Vestibular eye movements, reflexive movements that stabilize images on the retina during sudden movements of the head.
Fixation, a voluntary action that stabilizes the fovea on stationary objects or points of regard.
Optokinetic eye movements, reflexive eye movements that stabilize large moving visual fields on the retina.
Smooth pursuit eye movements that keep the fovea on moving targets. These movements are voluntary but cannot be performed without a visual target.
Nystagmus eye movements, reflexive movements that reset the eye during prolonged rotation or optokinetic eye movements thus preventing the eye from becoming pinned at the end of its range of travel.
Saccades, which allow the rapid shift of gaze from one target to another. Saccades can be voluntary or involuntary.
Vergence eye movements that shift the gaze in depth and keep both foveae aligned on a target. Most vergence eye movements are voluntary, however large field looming stimuli and sudden forward translations of the head can induce short latency vergence eye movements.
Because eye movements vary in several important ways, many other distinctions can be made. Eye movements can be divided into conjugate (movement is the same in both eyes) and disconjugate (the eyes move a different distance or direction). Using conjugate eye movements we can look from one target to another that differs only in direction. However, targets at different distances require the eyes to change their relative orientations; looking from one target to another at a different distance requires disconjugate movements (Straumann, 2007). Vergence eye movements are disconjugate movements that move the eyes between targets that vary in distance (Busettini, Davison, Gamlin, & Squire, 2009; Gamlin, 2002; Schor & Ciuffreda, 1983). Smooth pursuit is a conjugate eye movement that facilitates tracking of a small target against a background that differs in velocity from the target (Krauzlis, 2004) for example, watching a bird in flight against the clouds in the sky. This differs from saccades, the eye movements that are used to bring the fovea to a new target or location (McDowell, Dyckman, Austin, & Clementz, 2008). These rapid eye movements are ballistic in nature and are not generally subject to modification after initiation. The remaining eye movements, vestibular stabilization or optokinetic following responses, stabilize the view during movements of the head or the visual scene (Angelaki & Squire, 2009; Kheradmand & Zee, 2011; Mustari, Ono, & Squire, 2009).
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